1. Field of the Invention
The present invention relates to nonaqueous electrolyte battery.
2. Background Art
In recent years, the market of portable information equipment such as portable telephones (cellular phones) and small-sized personal computers is being expanded. With the progress of a reduction in size and weight of these equipment, there is an increasing demand for small-size and light-weight power supplies. Lithium ion rechargeable batteries having high energy density are extensively used in these portable equipment and are still being currently studied. The advance of technology in recent years has led to a reduction in size of a variety of equipment such as digital audio equipment and POS terminals. When the equipment is rendered portable by virtue of size reduction, a built-in battery, which can eliminate the need to use a power cord, is required instead of the conventional alternating-current power supply, and applications which require the use of rechargeable batteries are also being expanded. Also for information equipment such as personal computers and portable telephones in which rechargeable batteries have hitherto been used, an improvement in characteristics is always required. Required characteristics range widely, that is, not only capacity but also operation at elevated temperatures, safety at elevated temperatures, operation at low temperatures, high output, long-term stability and the like. For example, lead-acid batteries, nickel cadmium rechargeable batteries, and nickel hydrogen rechargeable batteries have hitherto been used as rechargeable batteries. However, there is still room for improvement in small size and light weight. Rechargeable batteries with a nonaqueous electrolyte have small size and light weight and have high capacity and thus have become used in the personal computers and portable telephones (cellular phones), as well as in digital cameras, video cameras and the like.
Batteries called lithium rechargeable batteries or lithium ion rechargeable batteries in which, for example, lithium-containing cobalt composite oxides, lithium-containing nickel composite oxides, and lithium-containing titanium oxides capable of occluding and releasing lithium, and carbonaceous materials are used as positive electrode materials and negative electrode materials, are among this type of rechargeable batteries with a nonaqueous electrolyte.
In this connection, rechargeable batteries with a nonaqueous electrolyte using a combustible organic solvent as an electrolysis solution have been put to practical use and commercialized. In order to improve stability under a higher-temperature environment or safety upon overcharge or breaking of the battery, however, studies have been made to use an ionic liquid, which has no flash point and is liquid at room temperature, as an electrolyte for enhancing the safety. Also for a primary battery which is easy to ensure safety as compared with the rechargeable battery because charging is not carried out, the application of ionic liquids has been studied, for example, from the viewpoint of improving the safety. For example, rechargeable batteries with a nonaqueous electrolyte using a lithium metal oxide in the positive electrode, using a lithium metal, a lithium alloy, or a carbonaceous material capable of absorbing and releasing a lithium ion, and an ionic liquid composed of a lithium salt, an aluminum halide, and a quaternary ammonium halide as an electrolyte are disclosed as rechargeable batteries having excellent safety, for example, in Japanese Patent Laid-Open No. 349365/1992. Further, rechargeable batteries with a nonaqueous electrolyte comprising a positive electrode, a negative electrode comprising a carbonaceous material capable of absorbing and releasing a lithium ion, a fluoride anion of an element selected from boron, phosphorus, and sulfur, and an ionic liquid composed of a quaternary ammonium ion and a lithium ion are disclosed as rechargeable batteries having excellent safety and improved cycle life and discharge capacity, for example, in Japanese Patent Laid-Open No. 86905/1999.
The ionic liquid, however, have problems remaining unsolved that the viscosity is higher than that of carbonate-type or other nonaqueous solvents used in conventional nonaqueous electrolyte batteries and, thus, the impregnation of the ionic liquid into the separator and positive and negative electrodes is difficult or impossible.